One of the biggest problems in mobile telephone systems is an efficient use of a limited radio capacity. In present systems, a certain amount of capacity is reserved for each user for a circuit-switched call during the entire radio link. When using packet-switched transmission, in which the data to be transmitted is typically generated in bursts, it is a waste of radio capacity to keep the radio capacity reserved all the time according to the greatest momentary data transmission need.
In systems using the code division multiple access (CDMA) method, in a downlink from a radio network subsystem to user equipment, different users use the same code tree in which the spreading codes used in the system are arranged mutually orthogonally. If a spreading code having a small spreading factor allowing a high transmission rate is reserved for one user, the code in question can reserve a large part of the capacity of the radio network subsystem in question, or its base transceiver station. In a code tree used by one base transceiver station, one sector of the base transceiver station can use sixteen mutually orthogonal sixteen-character-long spreading codes, for instance, in which case the entire capacity of the base transceiver station can at one moment be in use and a new user will not have access to any data transmission resources for a downlink.
On an uplink, this problem does not exist, since each user has access to the entire code tree of the base transceiver station. Various users are distinguished from each other with a scrambling code which is specific for each transmitter. For instance, a universal mobile telecommunication system (UMTS) using a direct-sequence wideband code division multiple access method (DS W-CDMA) can have 512 different scrambling codes and 256 different spreading codes on a downlink. On an uplink, the number of scrambling codes can be much higher, even millions of different codes. It is interesting to note that a base transceiver station normally uses only one scrambling code for each transmitter.
In present mobile telephone systems, the data transmission rates used for both speech and data have been relatively low, and it has been possible to keep the resource problem under control. In new mobile telephone systems, the data transmission rates used will be considerably higher than in the present systems due to the wireless remote use of various computer applications. The computer applications include various database applications, e-mail, WWW browser, etc.
For instance, the CDMA2000 system discloses a solution in which fundamental channels and supplementary channels are used. The fundamental channel transmits the signalling of the MAC (Medium Access Control) sub-layer of the data link layer, which indicates whether a supplementary channel with a higher transmission rate is used in addition to the fundamental channel. A problem with this solution is that it does not support fast changing of the channel transmission rate by changing the spreading code, since MAC sub-layer signalling is used to change the spreading code and it is a relatively slow process.